Color separators and recombiners are well known in the optical arts and have a number of applications, particularly for displaying or recording color images. In such imaging devices, a color separator accepts polychromatic light input and splits the light according to wavelength to provide two, three, or more spectral bands of light for modulation. A color recombiner is then used to combine the spectral bands of modulated light and form a full color image thereby.
Dichroic coatings provide a useful mechanism for performing color separation in a range of applications, offering advantages such as minimal light loss and relatively good spectral selectivity. Examples of various types of color separators that employ dichroic coatings on prism surfaces include the following:                U.S. Pat. No. 6,758,565 entitled “Projection Apparatus Using Telecentric Optics” to Cobb et al. describes the use of a dichroic separator in the form of a V-prism for separating white light into primary Red, Green, and Blue (RGB) color components;        U.S. Pat. No. 6,671,101 entitled “Color Combining Optical Element, Color Separation Optical Element, and Projection Type Display Apparatus Using Them” to Sugawara describes a color separating and combining optical component formed from three prisms that have dichroic surfaces positioned between them;        U.S. Pat. No. 6,517,209 entitled “Color-Separating Prism Utilizing Reflective Filters and Total Internal Reflection” to Huang describes another color separation apparatus using joined prisms with dichroic surfaces between them;        U.S. Pat. No. 6,956,701 entitled “Method and Apparatus for Combining Light Paths of Multiple Colored Light Sources Through a Common Integration Tunnel” to Peterson et al. describes an integrating apparatus for combining multiple color light paths for illumination in a projection system;        U.S. Pat. No. 5,098,183 entitled “Dichroic Optical Elements for use in a Projection Type Display Apparatus” to Sonehara describes the prismatic optical component using crossed dichroic surfaces that is known as the X-cube and is used in various types of imaging devices for separating white light into its RGB components;        U.S. Pat. No. 6,327,092 entitled “Cross Dichroic Prism” to Okuyama describes a variation on X-cube design wherein opposite outer faces are not in parallel;        U.S. Pat. No. 3,202,039 entitled “Optical System for a Color Television Camera” to DeLang et al. describes the Philips prism for color separation that has been employed in numerous projector designs, employing three prisms and a number of dichroic surfaces;        and,        U.S. Pat. No. 6,704,144 entitled “Color Separation Prism Assembly Compensated for Contrast Enhancement and Implemented as Reflective Imager” to Huang describes another embodiment of a three-prism arrangement for directing polychromatic light through various dichroic coatings to obtain separated RGB components.        
With such conventional X-cube and Philips prism embodiments, polychromatic light passes through one or more dichroic surfaces and is separated into its different spectral components thereby. In most cases, the same component, or a component similarly configured, can be used for both color separation prior to modulation and for recombining modulated light for final projection or other imaging.
Combinations of dichroic surfaces with prisms, such as those exemplified by the X-cube and Philips prisms, have a number of disadvantages. The X-cube, an arrangement that combines four separate prisms with each intersecting inner surface having a corresponding dichroic coating, has proven to be difficult to manufacture inexpensively. Slight misalignment of inner surfaces can result in problems such as color fringing. Mechanical tolerances and proper alignment can also present difficulties for the fabrication of Philips prisms and related three-prism devices. Moreover, even where these problems can be corrected, X-cube and Phillips prism solutions require that light paths be split in orthogonal directions or at some other relatively pronounced angle relative to the axis of incident light. The resulting spacing constraint prevents such devices from being used in any type of array, for example.
Another significant drawback of conventional color separation devices relates to characteristics of dichroic surfaces themselves. The spectral performance of a dichroic coating is strongly influenced by the angle of incident light. As the angle of incidence varies, the wavelengths of light that are transmitted and reflected also shift somewhat. Generally, dichroic surfaces are designed to work best with incident light over a small range of angles and are typically used for incident light at near-normal angles.
In many types of devices, including imaging apparatus, there is a need to provide a uniform field of light for modulation. To achieve a uniform field, one strategy is to employ a uniformizing element that spreads the light energy by homogenizing the angular distribution and shapes the light beam, without incurring loss of light. Imaging devices, such as those used for color image display or recording as noted above, typically use components such as an integrating bar, a fly's-eye lenslet array, a fiber optic faceplate or bundle, a diffuser, or similar optical device as a uniformizing element. As just one example, U.S. Pat. No. 6,919,990 entitled “Methods and Systems for Low Loss Separation and Combination of Light” to Anikitchev et al. describes the use of an integrating bar for redistributing an illumination beam to obtain more uniform distribution of the illumination energy in a display system.
While imaging applications typically require either spectral separation or recombination of light for forming color images, there are other applications in which both spectral separation and homogenization of light energy are advantageous. Among these applications are those in which energy is obtained from light, such as in sensing apparatus and in photovoltaic energy apparatus, such as solar energy panels, for example. Various photovoltaic materials respond differently to light at different wavelengths. For this reason, it can be beneficial for a solar energy apparatus to decompose sunlight into two or more spectral bands, directing each spectral band to an appropriate material that is optimized for providing energy at that spectral band. Where sunlight can be split into higher and lower spectral bands of longer and shorter wavelengths respectively, different photovoltaic cells can be utilized to increase the overall energy yield obtained from an amount of sunlight. A first photovoltaic cell can be optimized for lower energy light, that is light at longer wavelengths. A second photovoltaic cell can then be optimized for higher energy light, that is, light at shorter wavelengths.
While it is recognized that there would be benefits to apparatus and methods for efficient spectral splitting of light, conventional solutions fall far short of what is needed for lossless splitting of light at low cost and require that a separate component be provided in order to improve spatial distribution of light energy. Thus, there is a need for an apparatus that provides lossless spectral separation with light homogenization and allows compact packaging.